1. Field of the Invention
This invention relates to an improvement in an image recording method in which a light beam is on-off modulated based on an image signal obtained by scanning a continuous tone original, and a photosensitive recording medium is scanned with the on-off modulated light beam to record a dot image corresponding to the continuous tone original.
2. Description of the Prior Art
Conventionally, the image scanning and recording method of the type described above is conducted for example as described below. The descriptions below refer to the case where one picture element P of the image reproduced on a photosensitive recording medium F consists of the first to fourth cells C1 to C4, and each cell consists of the first to fifth lines L1 to L5 as shown in FIG. 1, which schematically shows the configuration of one picture element of a reproduced image. In this case, a single cell is formed by five scannings with the light beam. Usually, a laser beam is used as the light beam. Therefore, the image scanning and recording method is described below with respect to a laser beam used as the light beam for scanning. FIG. 1 is an extremely enlarged view of the picture element P, but actually the picture element P is generally of a size of about 225 .mu.m.times.225 .mu.m. Further, in the descriptions below, the cell and line are designated simply by reference characters C and L respectively, except where a particular cell or line is to be designated.
FIGS. 2A to 2C are explanatory views showing the manner of recording an image on a photosensitive recording medium by scanning the medium with a laser beam. In the conventional image scanning and recording method, a continuous tone image is first scanned with a laser beam and read to obtain an image signal S0 like that shown in FIG. 2A. The image signal S0 is compared with a triangular wave-like dot signal S1 to obtain a dot image signal S2 like that shown in FIG. 2B. As shown in FIG. 2A, the dot signal S1 consists of the first to fifth dot signals S11 to S15 which correspond to the respective lines L constituting the aforesaid cell C. The dot signals S11 to S15 are sequentially compared with the image signal S0 to form the first to fifth dot image signals S21 to S25 respectively as shown in FIG. 2B.
The dot image signals S21 to S25 thus formed are then input to a drive circuit of a light modulator to modulate the laser beam used for scanning the photosensitive recording medium. Thus the laser beam is modulated based on the dot image signals S21 to S25. This is done in such a way that, when the line L1 of the cell C1 is scanned with the laser beam, the first dot image signal S21 is input to the drive circuit of the light modulator. Therefore, the first line L1 is written on the photosensitive recording medium by being scanned with the laser beam which is on-off modulated based on the first dot image signal S21. Similarly, the second to fifth lines L2 to L5 are written on the recording medium by being scanned with the laser beams which are on-off modulated based on the second to fifth dot image signals S22 to S25. FIG. 2C shows the first cell C1 written on the recording medium.
With the image recording method in which dots are formed and recorded as described above, it is possible to reproduce a continuous tone original into a dot image having a continuous dot image gradation which is fairly satisfactory. With this method, however, gradation jumps in the dot area ratio of between about 1% and 3% occur five times in the 0% to 50% dot area region (when each cell consists of five lines) and five times in the 50% to 100% dot area region (when each cell consists of five lines). As a result, "streaks" develop at the gradation jump sections particularly in the skin color region of the reproduced image.
The reason why the gradation jumps occur in the 0% to 50% dot area region is explained below with reference to FIGS. 3A to 3C and 4A to 4C.
When the level of the image signal S0 drops from that shown in FIG. 2A to that coming upon the dot signal S11 as shown in FIG. 3A, a dot image signal S2 as shown in FIG. 3B is obtained by comparing the image signal S0 and the dot signal S1. The cell C1 is written on the recording medium as shown in FIG. 3C in accordance with the dot image signal S2. In comparing the cell C1 shown in FIG. 2C and that shown in FIG. 3C, it will be noted that the blackened section exists only on the second to fourth lines L2 to L4 in both cells, but the length of the blackened section differs therebetween (i.e. the length is longer in the case of the cell C1 shown in FIG. 3C). Thus, when the level of the image signal changes between the peaks of two dot signals whose levels are adjacent to each other, the change in the gradation of the image recorded on the recording medium is indicated by the change in the length of the blackened section on the line L. Accordingly, in this case, the gradation of the reproduced image changes analog-wise to yield the continuous gradation.
However, if the level of the image signal S0 slightly drops from that shown in FIG. 3A to that intersecting or slightly lower than the peak of the dot signal S11 as shown in FIG. 4A, a dot image signal S2 as shown in FIG. 4B is obtained by comparing the image signal S0 and the dot signals S11 to S15. From FIG. 4B, it will be understood that the on-state sections of the dot image signals S22 to S24 are slightly longer than those shown in FIG. 3B and, in addition, a short on-state section occurs in the first dot image signal S21. When the cell C1 is written on the recording medium based on the dot image signals S21 to S25, the lines L2 to L4 are blackened as shown in FIG. 4C and, in addition, a blackened section D is also formed on the first line L1. The blackened section D designates a dot having the minimum recordable size, which is stable on the recording medium F (the recording medium F used in the dot image field is a photosensitive material exhibiting high gamma characteristics, such as lith film) and which has a sufficient density (optical density: 0.5 or more) to be printed on a printing plate. This blackened section D is hereinafter referred to as the minimum blackened unit. In FIG. 3B, the pulse width of the dot image signal S21 obtained by the comparison between the dot signal S11 and the image signal S0 is very short, so that no blackened dot develops on the line L1 in FIG. 3C. Thus, the dot image signal S21 having a very short pulse width does not cause the blackened section to occur according to the pulse width of S21 if the ultrasonic modulator has low high-frequency response characteristics, if the laser beam is a Gaussian beam, or if a photosensitive material having high gamma characteristics such as lith film is used. As described above, in the cases of FIGS. 2A to 2C and 3A to 3C, the gradation continuously changes with an increase in the length of the blackened section. However, when the image signal S0 intersects the peak of the dot signal S1, the blackened section increases in increments of the minimum blackened unit D, i.e. digitally and discontinuously, and therefore the gradation also increases digitally and discontinuously. Accordingly, a gradation jump develops at this time.
For example, when the spot size of the laser beam is 25 .mu.m.times.25 .mu.m, the size of the minimum blackened unit is about 20 .mu.m.times.20 .mu.m. When the screen angle is 45.degree., two minimum blackened dots can be contained (at C1 and C2 in FIG. 1) in one picture element P (225 .mu.m.times.225 .mu.m). In this case, the dot area changes about 2%.
The reason why the gradation jumps develop in the 50% to 100% dot area region in now explained below with reference to FIGS. 15A to 15C and 16A to 16C.
When the level of the image signal S0 is at the position shown in FIG. 15A, a dot image signal S2 as shown in FIG. 15B is obtained by the comparison between the image signal S0 and the dot signal S1. The cell C is written on the recording medium as shown in FIG. 15C in accordance with the dot image signal S2. Then, if the level of the image signal S0 slightly drops from that shown in FIG. 15A to that shown in FIG. 16A, a dot image signal S2 as shown in FIG. 16B is obtained by the comparison between the image signal S0 and the dot signal S1. The cell C is written on the recording medium as shown in FIG. 16C in accordance with the dot image signal S2 shown in FIG. 16B.
In the case of FIG. 16B, the pulse width of the dot image signal S21 obtained by the comparison between the dot signal S11 and the image signal S0 is very short, and a minimum transparent or unblackened dot (hereinafter referred to as the minimum unblackened unit) develops on the line L1 in FIG. 15C. On the other hand, the minimum unblackened unit is abruptly blackened and disappears on the line L1 in FIG. 16C.
The aforesaid minimum unblackened unit has a sufficiently low fog density (optical density: 0.2 or less) for the printing of the printing plate and has the minimum size that can form an unblackened section.
When the dot image signal S21 has a very short pulse width as shown in FIG. 16B, the unblackened section corresponding to the pulse width of S21 is not reproduced and the whole line L1 is blackened if the ultrasonic modulator has low high-frequency response characteristics, if the laser beam is a Gaussian beam, or if a photosensitive material having high gamma characteristics such as lith film is used. Thus, in the case of FIGS. 16A to 16C, the blackened section increases in increments of the minimum unblackened unit. Accordingly, the gradation changes digitally and discontinuously, resulting in a gradation jump.
FIG. 5A is a graph showing the relationship between the image signal and the gradation of the image reproduced by use of the image scanning and recording method in accordance with the present invention and the conventional method. As shown by the solid line in FIG. 5A, ten gradation jumps occur in case the cell C consists of five lines L. The broken line in FIG. 5A indicates the ideal relationship between the image signal (input) and the gradation of the reproduced dot image (output).
The above-mentioned gradation jumps occur also in the case where the photosensitive material is based on silver chloride as described below.
Namely, when the dot percentage is 50%, the cells C1 and C2 of the picture element P shown in FIG. 1 are wholly blackened. In this case, the contacting point between the cell C1 and the cell C2 tends to become thickened and blackened, and the gradation jumps to an extent corresponding to the area of the thickened and blackened point.